Although the Ipr genotype was identified as a mutation in the death receptor gene, fas, 13 years ago, the source of the accumulating CD4+, CD8+, and unusual CD4-CD8- (CD4-8-) TCRab+ T lymphocytes remains an enigma. Autoimmune Ipr mice manifest no significant defect in thymic negative selection or peripheral T cell deletion following activation with antigenic peptides, superantigens, or infectious agents. This grant hypothesizes that the adenopathy of Ipr mice originates from unrestrained homeostatic proliferation of T cells. This is consistent with the observation that Ipr mice develop adenopathy even in an antigen-free environment. During homeostatic proliferation in RAG1-/- mice, donor Ipr T cells accumulate to much greater numbers than wild-type T cells, and give rise to CD4-8- TGRab+ cells. Since homeostatic proliferation is driven by low affinity TCR/self-MHC-peptide signals, this suggests that the function of Fas is to eliminate T cells receiving low affinity signals from self-antigens. This model also predicts that those CD8+ T cells receiving low intensity TCR signals will become CD4-8- TCRab+. The four aims are: Aim 1 will determine whether CD8+ T cells that make low avidity or low affinity TCR interactions with peptide/MHC preferentially give rise to CD4-8- TCRab+ T cells. First, TCR/MHC avidity will be decreased by transferring polyclonal CD8+ cells (wild-type, Ipr, or Bim-/-) into scid, b2m-/-scid, and TAP-/-scid mice. The second part will vary TCR/MHC affinity using Ova-responsive OT-I T cells transferred to TAP-/-RAG-/- mice expressing different Ova peptides of varying affinity for the OT-I TCR. Aim 2 approaches the same question by fixing the antigen (H-Y) and varying the TCR using anti-H-Y TCRb (H-Yb) T cells (wild-type, Ipr, or Bim-/-) and monitoring the presence of low affinity tetramer+ cells versus high affinity clonotype+ cells. Aim 3 examines how Fas can positively signal dendritic cells for effector functions that are important for homeostatic proliferation. Aim 4 tests whether homeostatic proliferation increases the frequency/functional of autoimmune T cells. Lymphopenic conditions following chemotherapy, irradiation, and HIV infection can provoke autoimmune conditions. Hence, the regulation of homeostatic proliferation is central to autoimmune mechanisms.